R-407C is an HFC refrigerant replacement for R-22, a HCFC refrigerant, in positive displacement systems such as new or existing residential and commercial air-conditioners and heat pumps. R-407C offers similar performance to R-22 and can be used to retrofit existing R-22 air-conditioning and refrigeration systems.
R-407C is a mixture of hydrofluorocarbons used as a refrigerant. It is a zeotropic blend of difluoromethane (R-32), pentafluoroethane (R-125), and 1,1,1,2-tetrafluoroethane (R-134a). R-32, R-125 and R-134a are present in the proportions 23%/25%/52% by weight (a ±2% tolerance is allowed for each of the components). R-407A is a mixture of 20% R32, 40% R125, and 40% R-134a, and is therefore R-407A is similar in many respects to R407C. Difluoromethane serves to provide the heat capacity, pentafluoroethane decreases flammability, tetrafluoroethane reduces pressure. R-22 is chlorodifluoromethane.
Most compressors require a lubricant to protect internal moving parts. The compressor manufacturer usually recommends the type of lubricant and proper viscosity that should be used to ensure acceptable operation and equipment durability. Recommendations are based on several criteria, which can include: lubricity; lubricant/refrigerant solubility; lubricant/refrigerant miscibility; compatibility with materials of construction; and thermal stability and compatibility with other lubricants. It is important to follow the manufacturer's recommendations for lubricants to be used with their equipment.
Lubricant return to the compressors is very important as well. One factor that affects this is the liquid-phase lubricant/refrigerant miscibility, particularly at evaporator temperatures. Miscibility is the ability of two liquids to mix and form a single liquid phase. Ideally, the lubricant-refrigerant pair have sufficient miscibility or mutual solubility to allow the lubricant to flow with the liquid refrigerant and return to the compressor in a single phase. Even if the lubricant/refrigerant pair are not miscible (two liquid phases form) in the evaporator, they may still have some degree of solubility. Solubility of refrigerant in lubricant lowers lubricant viscosity, which helps it flow through the evaporator and return to the compressor. This is why many refrigeration systems can operate properly, even though the lubricant and refrigerant are immiscible (yet partially soluble) at evaporation temperatures. Other factors, such as refrigerant vapor velocity and system geometry, play key roles in lubricant return. Overall, it is important to note that lubricant/refrigerant miscibility is helpful, but not necessarily essential for proper system operation.
In general, R-22 and mineral oil are miscible over most of the expected ranges of operating conditions for normal air conditioning and high to medium temperature operation. Miscibility is generally believed to aid lubricant return to the compressor. The miscibility of refrigerant and lubricant is determined by several factors: the relative amounts of refrigerant and lubricant present, the temperature, the chemical makeup of the lubricant and of the refrigerant, and the viscosity of the lubricant.
Therefore, it may be desirable in a retrofit of an existing system of this type to use a miscible lubricant, such as polyol ester lubricant, with R-407C refrigerant. In general, R-407C is miscible with polyol ester (POE) lubricant and immiscible with alkylbenzene and mineral oil lubricant.
Residual mineral oil or alkylbenzene left in a refrigeration system after a retrofit to any HFC refrigerant is performed is known to decrease the lubricant/refrigerant miscibility. This is one of the reasons why three oil changes are generally recommended when a system is being converted or retrofit to use HFC refrigerants and POE lubricants. Thus, a traditional recommendation is to replace all mineral oil or alkylbenzene lubricant with POE during a retrofit.
Emerson Climate Technologies (Copeland®) provided in 1995 & 2005 the following recommendations when conducting a changeover: “Copeland's only approved lubricants for use with R-407C are Polyol ester lubricants”; “Mineral oil lubricant, such as 3GS, cannot be used as the compressor lubricant. Copeland recommends the following lubricant choices: Polyol Ester (POE)”; “The mineral oil must be removed from the compressor crankcase. Hermetic compressors will have to be removed from the piping and the lubricant drained out through the suction stub. It is advisable to do an acid test on the lubricant removed”; “Those systems that have oil separators, oil reservoirs, oil floats and suction line accumulators must have the oil drained from them. If the liquid control device is going to be replaced, it is advisable that the suction line, liquid line, and evaporator coil be blown clean using properly regulated dry nitrogen”; “POEs are very hygroscopic. They will very quickly absorb moisture from the air once the container is opened. Once the lubricant is added to the compressor, the compressor should be quickly installed. Like an open container, an open compressor with POE will absorb moisture. Add the correct amount of lubricant to the compressor. On systems using enhanced surfaces in the heat exchanger, excessive mineral oil can adversely affect the heat transfer due to logging. Therefore, it is desirable to have no more than one percent mineral oil in systems employing these types surfaces.” lvhvac.com/cope_bulletins/95-14.pdf.
Dupont (2004) advises that no more than 5% mineral oil remain in the refrigeration system after a changeover from R-22 to R-407C. (www2.dupont.com/Refrigerants/en_US/assets/downloads/h70004_Suva407C_retrofit_guide.pdf)
Similarly, Tecumseh advised in 2009 (www.air-innovations.nl/web/pdf/GUIDELINES-FOR-UTILIZATION-OF-R407C.pdf):                Synthetic Lubricants: Polyol Esters (POEs) And Polyvinyl Ethers (PVEs)        
A) Miscibility                1. Miscibility is the ability of the lubricant and the refrigerant to mix. This miscibility is a very important factor in providing proper heat transfer and in returning lubricant to the compressor in a refrigeration system over its range of operating temperatures.        2. R407C and Mineral oils (MO) are not miscible        3. Polyol Ester (POE) as well as Polyvinyl Ether (PVE) oils and R407C are miscible.        4. R407C and Synthetic Alkylate (SA) oils are partially miscible . . . .        
C) Compatibility . . . .                2. While polyol ester and polyvinyl ether oils are compatible with mineral oils, they should not be indiscriminately mixed with mineral oils in R407C refrigerant systems. This practice could result in the inability of the oil to return to the compressor and/or reduce heat transfer performance in the evaporator. However, small amounts up to 5% of mineral oil are acceptable in field retrofit situations (1% or less is preferred).        
Ennio Campagna et al., “The Use of R-407C: Applications and Guidelines”, makina.beun.edu.tr/eskisite/akademik_kadro/meyriboyun/mak723/The_Use_of_R-407C.pdf, states:
1) General Considerations for HFC Refrigerants                Systems which use R407C normally have compressors charged with a miscible lubricant such as POE (polyolester). There are many manufacturers and grades of these lubricants. R-407C is generally not used with mineral or alkyl-benzene (i.e. hydrocarbon based) oils as the miscibility is not adequate to ensure satisfactory oil return to the compressor. It is very important that, when servicing a system and oil is needed, the correct oil be used. Seek guidance from the system or compressor manufacturer on which specific oil to use in a maintenance situation.        b) System contamination with mineral oils, or with R22, can have a serious effect on performance and system reliability. Systems which use enhanced tube heat exchangers (evaporators and condensers) are particularly vulnerable to performance degradation arising from the presence of HC based oils. It is strongly recommended that dedicated service gauge sets are used for R407C to prevent inadvertent contamination.        
Similarly, Herronhill provides general guidelines to assist in the replacement of R22 condensing units with equivalent R407C units (2004, www.heronhill.co.uk/technical-information/40/converting-r22-systems-to-r407c), stating “Condensing units built and supplied by IMI Air Conditioning Ltd. (now Marstair Ltd) before 1999, for use with R22 will contain mineral oil. R407C units will contain polyolester oil (POE). Check the compressor label to confirm the type of oil in the system. For safe system operation it is essential to remove the mineral oil from the existing system, as R407C is not compatible with mineral oil. On completion of the change over the concentration of mineral oil remaining in the system should not exceed 5% of the total oil charge.
Sundaresan, S. G.; Pate, M. B.; Doerr, T. M.; and Ray, D. T., “A Comparison of the Effects of POE and Mineral Oil Lubricants on the In-Tube Evaporation of R-22, R-407C and R-410A” (1996). International Refrigeration and Air Conditioning Conference. Paper 322. docs.lib.purdue.edu/iracc/322, describe tests on the heat transfer efficiency of R-22, R-407C and R-410A on smooth and fined tubes, when neat, mixed with mineral oil, and when mixed with POE oil. POE-mineral oil mixtures were not tested. The heat transfer efficiency was higher for R-407C mixed with POE than with mineral oil.
Some more recent consideration of the choice or lubricant considers mixtures of lubricants, including mineral oil (MO) and polyol esters (POE). For example, The News, (Peter Powell, Dec. 3, 2012, www.achrnews.com/articles/print/121529-retrofit-round-up-plenty-of-options) states:
Retrofit Round-Up: Plenty of Options                Because no one HFC refrigerant can be retrofitted efficiently into the entire range of R-22 equipment, a wide and sometimes changing variety of HFCs have entered the market. The NEWS contacted a number of refrigerant manufacturers asking them to discuss some of their most popular HFCs for R-22 retrofits, the range of applications, and servicing tips.        While this can provide a guideline, The NEWS urges readers to contact refrigerant manufacturers directly to discuss the specific R-22 system being considered for retrofit and what HFC refrigerants might best work in such applications. This listing is in alphanumerical order according to ASHRAE designations.        
R-407A                Gus Rolotti, technical marketing director of Arkema's North American fluorochemicals business, described R-407A as a refrigerant for low- and medium-temp refrigeration. “Both R-407A and R-407C are well proven in the industry and provide good performance coupled with a lower GWP. They require the use of a POE oil.” Because of that, there needs to be a changeout of the mineral oil in an existing R-22 system.        Noted Rolotti, “First, fix any leaks and identify components that may need to be replaced or fixed. Recover the R-22 completely, repair/replace any components as identified earlier and pull the system into a deep vacuum. Remove the required amount of mineral oil either by flushing with an external agent or using the R-22 from the system. Once most of the mineral oil is removed, replace it with POE oil and pull the system into a deep vacuum again. Refill with the appropriate amount of the retrofit refrigerant and run the system to set up controls and operating conditions. It is recommended that the filter drier is always replaced when the system is opened to ensure that there is fresh desiccant available to dry the system.”        
R-407C                Arkema's Rolotti considers R-407C a choice for air conditioning. Retrofit procedures are similar to those with R-407A refrigerant.        In his comments regarding R-407C, Ron Vogl, technical marketing manager for Honeywell Refrigerants, described it as a choice for residential and light commercial a/c applications. The retrofit process for R-407C, he said, involves recovering R-22. Then, regarding oil, Vogl said, “POE is recommended. If existing MO is used, the addition of some POE to the system is required for proper oil return.” The use of the refrigerant requires a minimum of 20 percent POE for close-coupled a/c systems with no liquid receivers. No TXV change required, but minor adjustments may be needed. Replace any O-rings, seals, and other elastomers used in the system. In most air conditioning and heat pump systems this only applies to the Schrader valve seal and cap seal material. The next step is to evacuate and recharge. (Pounds of 407C=pounds of original refrigerant×70.6 divided by 73.9.) Then adjust system safety and operating controls; label system indicating refrigerant and charged lubricant.        
National Refrigerants, states (date unknown, www.nationalref.com/pdf/R22_Alternatives.pdf):                Will the blends work with mineral oil?        HFC Refrigerants with Mineral Oil        They don't mix so the mineral oil will form a separate layer in the receiver or other high-side liquid hold up.        Mineral Oil will remain thick on the low side and not be returned back to the compressor.        Will the blends work with mineral oil?        Hydrocarbons (HC) in Blends        HC soaks into mineral oil and thins it so it will circulate easily around the system.        The thinned mineral oil will move easily on the low side.        The mineral oil/HC mixture will still not mix with liquid HFC on the high side of the system. A layer of oil will still form in the receiver or other liquid hold-up.        
Partial POE Retrofit                Some amount of mineral oil is replaced with POE (does not require 3 flushes or less than 5% mineral oil).        The POE mixes well with the mineral oil, and the resulting mixture dissolves somewhat into liquid HFC.        The oil mixture will be carried by the liquid HFC out of the receiver.        National Refrigerants testing has shown that as little as 10% POE in mineral oil will circulate as much as an older compressor will pump out into a system.        These tests have also shown that there is very little difference in oil solubility with all of the blends on the market, regardless of the hydrocarbon content in the blend. Partial POE works with all of the blends, but hydrocarbons do not appear to help with liquid solubility of mineral oil with HFCs.        
Therefore, there has been some evolution as to whether mineral oil can remain in the refrigeration system after a retrofit, with the permissible proportion of mineral oil increasing. In particular, National Refrigerants proposes a partial POE retrofit, with greater than 5% mineral oil remaining. However, the existing recommendations appear to require empirical testing for cases other than less than 5% mineral oil remaining.
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